Method for forming a high strength silicon nitride

ABSTRACT

A method for forming a improved silicon nitride material characterized by high mechanical strength at high temperatures comprising a combination of approximately 0.5-6.0% by weight of strontium oxide, 2.0-12.0% by weight of yttrium oxide and the balance silicon nitride. The improved material is densified by either an encapsulated hot isostatic pressing method or an unencapsulated sinter/HIP method and can be utilized to form near net shape articles.

This invention was made with Government support under Contract No.NAS3-24385 awarded by the National Aeronautics and Space Administration.The government has certain rights in this invention.

This is a division of application Ser. No. 165,654 filed Mar. 8, 1988,now U.S. Pat. No. 4,870,036.

BACKGROUND OF THE INVENTION

The present invention relates to a dense silicon nitride material havinga novel composition and, in particular, relates to an improved siliconnitride material incorporating strontium carbonate, as a green statematerial, and yttrium oxide which are used as sintering aids to producethe dense silicon nitride material having high strength over a widerange of temperatures.

Sintered silicon nitride materials are well-known in the art. However,silicon nitride materials are usually limited in that the materials donot exhibit high strength and long stress rupture life when utilized ina high temperature (greater than 1000° C.) environment. It is known thatsintering aids have been utilized in order to increase the density ofthe sintered material which in turn increases the mechanical strength.It is also known that the incorporation of yttrium oxide (Y₂ O₃) is andremains one of the most utilized sintering aids to increase theproperties of sintered silicon nitride materials. Furthermore, it isknown to use strontium oxide (SrO) as a sintering aid with siliconnitrides, however, strontium oxide is unstable and reacts with theatmosphere and therefore degrades prior to the final formation of thesilicon nitride material. In addition, strontium oxide is much moreexpensive than strontium carbonate which is stable.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to produce a densesilicon nitride material which has improved mechanical strength andstress rupture life at high temperatures.

It is another object of the present invention to produce a siliconnitride material which has been processed using an appropriatecombination of strontium carbonate and yttrium oxide as startingsintering aids.

More specifically, in accordance with the present invention, there isprovided a densified silicon nitride having a final compositioncomprising (i) 82.0-97.5% silicon nitride (ii) 0.5-6.0% of strontiumoxide and (iii) 2.0 to 12.0% of yttrium oxide. The silicon nitride ismade by mixing the three constituents; silicon nitride, yttrium oxideand strontium carbonate along with an appropriate organic binder, ifnecessary, for forming the shape. The article can then be formed by anyappropriate method such as hot pressing, cold isostatic pressing,injection molding, extrusion, uniaxial die-pressing or slip casting.Thereafter, the article is thermally treated in a vacuum in order todecompose the strontium carbonate into carbon dioxide and strontiumoxide. The article is then cooled to room temperature in the vacuum thenstored in a low humidity environment such as a dessicator which keepsthe article from contacting moisture. Densification by hot isostaticpressing is achieved by encapsulating the article in a metal (e.g. Nb)can under vacuum utilizing electron beam welding or other encapsulationtechnique, and heating to a temperature greater than 1600° C. in aninert atmosphere while pressurized to 192.9 MPa (28 KSI) (hotisostatically pressed) before cooling to room temperature and ambientpressure. The article can also be hot isostatically pressed to highdensity using other encapsulants such as high temperature glass fritsystems. In addition, the article can also be densified by sinter/hotisostatic pressing process which eliminates the need to use anencapsulant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the present invention, a family of dense silicon nitridematerials are prepared from mixtures comprising silicon nitride, yttriumoxide and strontium carbonate. The mechanical characteristics of thesilicon nitride material is improved in that a dense body which is madefrom approximately 0.5-6.0 wt. % of strontium oxide (after conversionfrom strontium carbonate) 2.0-12.0 wt.% yttrium oxide, and the balancesilicon nitride is highly densified and has an improved mechanicalflexure strength at high temperatures. FIG. 1 compares the flexuralstrength of one of the improved silicon nitride materials (8 wt.% Y₂ O₃,2 wt.% SrO and 90 wt. % Si₃ N₄ densified by Nb encapsulation) with thestate of the art silicon nitrides. This densified silicon nitride hasthe flexural strength and other typical properties shown in Table I. Asshown within FIG. 1, the improved silicon nitride has significantlyhigher flexural strength over a range of temperatures.

                  TABLE I                                                         ______________________________________                                        NEW HIGH TEMPERATURE Si.sub.3 N.sub.4 *                                       (Densified by Nb Encapsulation/HIP)                                           PROPERTY                                                                      Property         Measured Value                                               ______________________________________                                        4-point flexural strength,                                                                     889.5 (129.1)-22° C.,                                 MPa (KSI) - temperature °C.                                                             706.7 (102.5)-1232° C.,                                                580.8 (84.3)-1399° C.                                 Young's Modulus, GPA                                                                           306.00                                                       Shear Modulus, GPA                                                                             120.00                                                       Hardness, KG/MM.sup.2                                                                          1405                                                         Fracture Toughness,                                                                            5.54                                                         MPa M.sup.1/2 (Indentation                                                    Method)                                                                       Thermal Expansion COEF,                                                                        4.3 × 10.sup.-6 (Ave 50°-1000° C.)       (Cm/Cm/°C.)                                                            ______________________________________                                         *8 wt. % Y.sub.2 O.sub.3, 2 wt. % SrO and 90° wt. % Si.sub.3           N.sub.4                                                                  

Table II shows the results of thirteen different densified articlecompositions and their corresponding densified properties. Thesearticles were all densified by the Nb encapsulation and HIP process.Each of these articles were fabricated using green-body articles havinga composition of 0.5-6.0% (after decomposition from strontium carbonate)by weight of strontium oxide, 1.8 to 12.0% by weight yttrium oxide andthe balance silicon nitride. The starting material for strontium oxideis strontium carbonate at a weight percentage of from 0.7-8.6%.Strontium carbonate is utilized in order to stabilize the sintering aidin the green forming stage and thereby retain the integrity of thearticle through densification of the final silicon nitride material.

                  TABLE II                                                        ______________________________________                                        DENSIFIED ARTICLE COMPOSITION AND STRENGTH                                    (Nb Encapsulation/HIP)                                                        Composition                                                                   Si.sub.3 N.sub.4                                                                    Y.sub.2 O.sub.3                                                                       SrO            Average                                          (Wt.  (Wt.    (Wt.   Density,                                                                              Flexure strength, MPa (KSI)                      %)    %)      %)     (g/cc)  (22° C./1232° C./1400°                                   C.)                                              ______________________________________                                        92    7       1      *       * / * / *                                        91    8       1      3.30    554.0/584.3/598.7                                                             (80.4)/(84.8)/(86.9)                             91    7       2      3.30    785.5/643.5/559.5                                                             (114.0)/(93.4)/(81.2)                            90.7  7.7     1.6    3.30    701.4/624.9/662.1                                                             (101.8)/(90.7)/(96.1)                            90    9       1      3.32    514.0/487.1/558.1                                                             (74.6)/(70.7)/(81.0)                             90    8       2      3.31    889.5/706.3/580.8                                                             (129.1)/(102.5)/(84.3)                           90    7       3      3.31    853.0/535.4/ *                                                                (123.8)/(77.7)/ *                                89.7  8.7     1.6    3.32    * / * / *                                        89.7  7.7     2.6    3.31    837.1/ * / *                                                                  (121.5)/ * / *                                   89    10      1      *       * / * / *                                        89    9       2      3.32    722.8/668.3/626.3                                                             (104.9)/(97.0)/(90.9)                            89    8       3      3.32    673.2/535.4/465.1                                                             (97.7)/(77.7)/(67.5)                             89    7       4      *       * / * / *                                        ______________________________________                                          * Not available                                                         

Green-body articles having a composition in the range given above can bedensified in one of two ways. The first process requires an encapsulantwhile the second does not, each process can be used for near net shapeforming of articles.

The first densification process is referred to as the encapsulation/hotisostatic pressing (HIP) process and comprises the steps of mixing thethree constituents having the desired particle size by milling or othertechniques. The mixed material is formed into the desired shape eitherby cold isostatic pressing, uniaxial die-pressing, injection molding,compression molding, extrusion or slip casting. In the case of coldisostatic pressing the milled ceramic powder is placed within a rubberdie (or a polymeric envelope) and subjected to pressures of 275.6-413.4Mpa (40-60 KSI) at room temperature. In the case of uniaxial pressingthe ceramic powder (with or without a small amount of organic binder) iscompressed to high pressure 17.2-68.9 MPa (2.5-10.0 KSI) in a metal die.In the case of injection molding, compression molding or extrusion, anorganic binder at an appropriate percentage is blended with the ceramicpowder to form a molding feed stock (or molding mix) and then moldedinto a shaped article using an injection molder, compression molder orextruder. The organic binder in the article is subsequently removed byan appropriate thermal cycle. In the case of slip casting, the milledceramic powder is dispersed in water with the aid of a dispersant toform a low viscosity slurry or slip. This slip is then cast into a moldto form the shaped article, which will be dried prior to furtherprocessing. The formed green body, by any one of the above processes, isheated in a vacuum at 1100°-1250° C. for 3-10 hours. The heating stepresults in the decomposition of the strontium carbonate to the strontiumoxide. Thereafter, the formed green body is cooled to room temperaturewhile still in the vacuum in order to protect the green body fromcontact with moisture. The green body is then enclosed in an encapsulant(metal can or glass) while remaining under the vacuum. Once sealedwithin the encapsulant, the green body is subjected to temperatures ofapproximately 1600°-1900° C. and pressure of 10.3-206.7 MPa (1.5-30 KSI)for 1 to 8 hours in an inert atmosphere. Thereafter, the densifiedarticle is allowed to cool to room temperature.

The flexural strengths of thirteen compositions from within thespecified range, densified by the Nb encapsulation/HIP method havealready been listed in Tables I and II. One of the thirteen compositions(8 wt. % Y₂ O₃, 2 wt. % SrO, and 90 wt. % Si₃ N₄), was also densified bythe glass encapsulation/HIP method, which is a near-net-shape process.The strength of this material approaches that of the material densifiedby the Nb encapsulation/HIP method (Table I). Table III shows theflexural strength as well as other properties of the glassencapsulated/HIP'ed material.

                  TABLE III                                                       ______________________________________                                        SILICON NITRIDE*                                                              (Densified by Glass Encapsulation/HIP Process)                                TYPICAL PROPERTIES                                                            ______________________________________                                        Density               3.31 g/cc                                               Flexure Strength (4 pt.)                                                      22° (72° F.)                                                                          798 Mpa (115.8 ksi)                                     1232 (2250° F.)                                                                              652 MPa (94.6 ksi)                                      1399 (2550° F.)                                                                              471 MPa (68.4 ksi)                                      Weibull modulus       13                                                      Fracture Toughness (Chevron Notch)                                            22° C. (72° F.)                                                                       6.05 MPa M 1/2                                                                (5.51 ksi in 1/2)                                       Young's Modulus       306 GPa (44.3 Msi)                                      Shear Modulus         120 GPa (17.4 MSi)                                      Stress Rupture Life (4 pt.)                                                   1204° C. (2200° F.) 482.3 MPa (70 KSI) ≧ 150 hr          1260° C. (2300° F.) 344.5 MPa (50 KSI) ≧ 230 hr          1260° C. (2300° F.) 413.4 MPa (60 KSI) ≧ 100 hr          1371° C. (2500° F.) 206.7 MPa (30 KSI) ≧  90 hr          Thermal Expansion Coefficient                                                 50-1000° C.    4.3 × 10.sup.-6 /C.                               ______________________________________                                         *2 wt. % SrO, 8 wt. % Y.sub.2 O.sub.3 and 90 wt. % Si.sub.3 N.sub.4      

The second densification process is referred to as the sinter/hotisostatic pressing (Sinter/HIP) process and as stated does not requirean encapsulant and is better suited to forming near net shape articleseconomically. With this process, the shaped green body can be formed bythe methods described above, i.e. slip casting, injection or compressionmolding, cold isostatic pressing; etc. and vacuum heat treated todecompose the strontium carbonate to strontium oxide while removing thecarbon dioxide.

The shaped green body is then placed in a crucible or container andburied in a mixture of powders of silicon nitride (Si₃ N₄), yttriumoxide (Y₂ O₃), aluminum oxide (Al₂ O₃) and boron nitride (BN). Thecrucible is then placed in a furnace and heated in nitrogen atmosphereat a temperature in the range of 1800°-1900° C. and pressure in therange of approximately 0.7-2.1 MPa (100-300 psi) for 2 to 6 hours.Thereafter, the pressure is increased to a pressure in the range ofapproximately 10.3-206.7 MPa (1.5-30 KSI) for a period of 1 to 8 hours.Following the Sinter/HIP process, the article is allowed to cool toatmospheric conditions.

Table IV shows net shape articles which have been formed by a variety ofgreen-forming processes; slip casting, injection molding or coldisostatic pressing, and densified utilizing either the encapsulation/HIPor sinter/HIP process. As disclosed, the final net shape articles canrange from a simple billet to a complex shaped turbocharger turbinerotor which includes a wide variation of cross-sectional sizes.

                  TABLE IV                                                        ______________________________________                                               Green Forming                                                                             Densification   Density                                    Shape  Process     Process         (g/cc)                                     ______________________________________                                        Billets                                                                              Cold Isostati-                                                                            Metal can & glass                                                                             3.30-3.31                                         cally pressed                                                                             encapsulated/HIP                                           Test bars                                                                            Injection   Sinter/HIP      3.27-3.31                                         Molded      glass encapsulation/HIP                                    Turbo- Slip Cast   Glass encapsulated/                                                                           3.28-3.30                                  charger            HIP                                                        Turbine                                                                       Wheels                                                                        Stator Injection   Glass encapsulated/                                                                           3.27-3.30                                  vanes  Molded      HIP                                                        Plates Slip Cast   Glass encapsulated/                                                                           3.29-3.31                                                     HIP and Sinter/HIP                                         ______________________________________                                    

It is presently believed that the materials used, herein i.e. siliconnitride, yttrium oxide, and strontium carbonate reduced to strontiumoxide, when mixed within the specified weight percentages, and densifiedaccording to the described processes, combine to form a grain boundarycomposition having superior flexure strength and adhesion to the siliconnitride grains. In particular, mixtures having bulk compositions of89-91 weight % silicon nitride, 7-10 weight % yttrium oxide and 1-3weight % strontium oxide have been processed and bulk flexure strengthsfrom 560 to 662 MPa at 1400° C. have been observed.

While a number of specific test compositions, processes and near netshape articles have been set forth and described, it is understood thatthese are provided by way of example only and that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims:

What is claimed is:
 1. A method of densifying a silicon nitride green body from a mixture of by weight, 0.7-8.6% strontium carbonate, 1.8-12.0% yttrium oxide and the balance silicon nitride, said method comprising the steps of:encapsulating said green body under vacuum; decomposing said strontium carbonate to strontium oxide and carbon dioxide; and hot isostatically pressing the green body to form a densified article.
 2. The method of claim 1 wherein the step of encapsulating comprises applying a glass coating over the green-body.
 3. The method of claim 1 wherein said step of encapsulating comprises enclosing the green-body in a metal can and sealing said can.
 4. The method of claim 1 wherein said step of hot isostatically pressing comprises heating the encapsulated green body to 1600-1900° C. and isostatically pressing the green body at 10.3-206.7 MPa (1.5-30 KSI) for 1 to 8 hours.
 5. A method of producing a densified silicon nitride based article comprising the steps of:mixing silicon nitride with 0.7%-8.6% strontium carbonate by weight and 1.8% -12.0% yttrium oxide by weight; forming the mixture into a desired shape; heating the shape in a vacuum or inert atmosphere to decompose the strontium carbonate into a strontium oxide; and hot sintering the shape under pressure to form the densified article.
 6. The method of claim 5 wherein the step of hot sintering includes the steps of vacuum encapsulating the shape and hot isostatically pressing the encapsulated shape.
 7. The method of claim 5 wherein the step of mixing silicon nitride with strontium carbonate and yttrium oxide includes mixing with an organic binder. 